Multisubband photoluminescence in p-type modulation-doped ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}/\mathrm{GaN}$ superlattices

Photoluminescence spectra from p-type modulation-doped ${\mathrm{Al}}_{0.20}{\mathrm{Ga}}_{0.80}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ superlattices with 10-nm well width show multiple, well resolved, interband transitions between quantum-confined states. In addition to the ground-state transition, a number of excited-state transitions are observed. The observation of multiple peaks is attributed to the inverse dependence of subband population and oscillator strength on energy. The relative strength of the peaks strongly changes with excitation intensity. At low excitation intensity, the spectra display only the ground-state transition. At higher excitation intensity, excited-state transitions become dominant. At high excitation intensities, the dominant transition occurs at energies about 500 meV above the electron ground-state to hole ground-state transition. Self-consistent calculations are used to assign transition energies, lifetimes, and rates to each photoluminescence line. Theoretical and experimental transition energies are in excellent agreement. We attribute the excellent optical properties to the modulated doping of the structure, which consists of doped barriers and undoped well layers. Our calculations also show an average recombination lifetime of 50 ns at high excitation intensities, despite the large quantum-confined Stark effect. The changes of the photoluminescence spectra can be explained via the effects of band filling and oscillator strengths at higher excitation intensity.